The Effect Of Cannabis On Driving Ability

There is still a lot of uncertainty about the effect of cannabis on driving ability. Blowing while driving is in any case not allowed. But it is hardly checked.

In 2011, the government passed a bill on drugs in traffic. This contains limited values ​​for drugs in the blood. Drivers’ blood may contain a maximum of 3.0 micrograms of THC per liter. People who have smoked a joint are therefore not allowed to drive for two to three hours.

Regular users

But when someone regularly smokes weed, the amount of THC in the blood can rise sharply. If you smoke a joint every day, it can take up to four days before you are legally allowed to drive again. In practice, this means that everyday users are never allowed to drive. Technically, even cycling should no longer be allowed.

CBD and traffic

With CBD it is different. No limit value has been included for CBD. Therefore, it is in principle allowed to drive after taking CBD oil. However, you have to be careful with that. Most bottles of CBD contain a small amount of THC. The bottles of oil from the store contain a maximum of 0.2% of the psychoactive stuff. But homemade CBD oil sometimes undetected contains much more THC. For that reason, it is difficult to say whether you are actually allowed to drive while taking CBD.

It is not known whether the substance CBD has an influence on driving ability. Studies into the effect of drugs in traffic usually only look at the substance THC. But it is to be expected that the influence of CBD on the reaction speed is nil because the substance has no psychoactive effect.

Traffic Control

The police naturally want to be able to check whether people are breaking the law. That is why the bill from 2011 includes that saliva tests may be used as a means of control. The saliva tests can therefore be used for more than five years according to the law. Yet not a single police officer in the Netherlands uses saliva tests.

That has everything to do with money. The Ministry of Security and Justice has to come up with the money to purchase the test devices. To date, that has not happened, but that will soon change. Minister Ard van der Steur has indicated that he will introduce the saliva tests in mid-2017.

At the moment, agents can only determine whether someone has used cannabis through a blood test. For this, the motorist must be taken to the police station and detained for some time. Of course, this only happens if there is a strong suspicion of drug use, for example when someone performs dangerous maneuvers.

Research into effect

Whether people perform dangerous antics after they have smoked has been extensively researched by various authorities. For example, the European Monitoring Center for Drugs and Drug Addiction conducted five years of research into the effect of different drugs on driving ability. The so-called DRUID Project came in 2011 with recommendations for the European Member States. The results of the study state that cannabis has an influence on driving ability, but at most as much as the permitted amount of alcohol.

The DRUID Project has drawn up four risk categories in order to properly compare the effect of different types of drugs. Cannabis falls under the ‘Slightly Increased Risk’ category, according to the study, which means 1 to 3 times as much risk of an accident. By comparison, a large amount of alcohol in the blood can increase the risk of an accident by 200 times.


During the DRUID Project, it emerged that cannabis users adjust their driving behavior as soon as they are stoned. They drive slower and more carefully. Such behavior only partially compensates for slower reaction speed, loss of motor control, and reduction in memory. The study did find a clear difference between experienced and inexperienced cannabis users. The real stoners sometimes showed no reduction in driving ability at all.

But a Ph.D. study from the University of Groningen is a lot more critical of the effect of cannabis on traffic. According to PhD student Janet Veldstra, it is undeniable that cannabis has a negative effect on driving skills, regardless of how experienced the user is. Her research acknowledges that cannabis users are driving more cautiously, but “that compensatory behavior is insufficient to counteract the effects of the drug on driving safety.”

Difference results

The conclusions of the EMCDDA study are therefore somewhat milder than those of the University of Groningen. Part of the difference could be explained by the different test methods. Test subjects in Groningen were given synthetically produced THC, while participants of the DRUID Project smoked a ‘normal’ joint.

Flowers of the cannabis plant contain more cannabinoids than THC alone. The other cannabinoids are known to influence the effect of THC. They may also have an effect on driving ability. Whether cannabinoids such as CBD , CBN or CBG have a positive influence on the reaction rate cannot be concluded from the studies. But one thing was certain: none of the subjects started driving better after they ingested THC.

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